1. Field of the Invention
The present invention relates to a semiconductor device fabricating method having a gettering step, which can efficiently remove a pollution heavy metal in a silicon substrate.
2. Description of the Related Art
In the fabrication of semiconductor devices, a pollution heavy metal, such as iron or nickel, which is introduced into a substrate leads to an increase in leakage current at the junction between the substrate and an electrode, and a reduction of charges stored in the charge holding portion of a DRAM, and so forth, which adversely affect the perfomance of semiconductor devices. One of known ways of reducing such heavy metal pollution is to make the semiconductor-device production line clean to thereby reduce the amount of a pollution heavy metal introduced during fabrication of semiconductor devices. Another way which has been proposed to reduce such pollution is a gettering step capable of eliminating a pollution heavy metal introduced in a substrate, from the substrate (Unexamined Japanese Patent Publication No. Sho 61-159741).
FIGS. 1A through 1D are cross-sectional views illustrating a step by step semiconductor device fabricating method having a conventional gettering step. As shown in FIG. 1A, first, a silicon substrate 301 is oxidized to form silicon oxide films 302a and 302b at its front and rear surfaces.
Next, as shown in FIG. 1B, only the silicon oxide film 302b at the rear surface of the silicon substrate 301 is removed to expose the rear surface of the silicon substrate 301.
Then, as shown in FIG. 1C, a polycrystalline silicon film 303 for a gate electrode is grown on the silicon oxide film 302a and a polycrystalline silicon film 304 is grown on the rear surface of the silicon substrate 301 as one step in the normal semiconductor device fabrication. The growth of the polycrystalline silicon film 304 gives stress to the substrate 301, thus crystal defects are produced in the substrate 301.
Thereafter, as shown in FIG. 1D, an impurity 305, such as phosphorus is introduced into the polycrystalline silicon film 303 for the gate electrode to impart conductivity to this polycrystalline silicon film 303, and it is also introduced into the polycrystalline silicon film 304. Then, the impurity introduced in the polycrystalline silicon film 304 is diffused into the substrate 301 from the polycrystalline silicon film 304. As a result, SiP or the like is produced in the substrate 301, causing deformation in the substrate 301. Thereafter, the substrate 301 is subjected to a heat treatment as one step in the normal semiconductor device fabrication.
The above-described process can permit a heavy metal, such as molybdenum or tungsten, present in the substrate 301 to be caught in the crystal defects and deformation as well as in the polycrystalline silicon film 304.
Even when a semiconductor device is manufactured by the above-described fabrication method, however, it is not possible to sufficiently getter the pollution heavy metal from the substrate 301 for the reason given below. In accordance with the recent increase in integration and miniaturization of semiconductor devices, the polycrystalline silicon film 303 for the gate electrode formed on the front surface of the silicon oxide film 302a on the silicon substrate 301 is thinner. The polycrystalline silicon film 304 formed at the rear of the substrate 301 is also thinner, thus reducing the stress which is given to the substrate 301. As a result, the amount of crystal defects formed in the substrate 301 is reduced, reducing the gettering effect. This is because the polycrystalline silicon film 304 is a thin film, and the diffusion of an impurity like phosphorus in the polycrystalline silicon film 304 reduces the amount of grain boundaries in the polycrystalline silicon film 304. Further, according to the conventional fabrication method, the heat treatment to getter a pollution heavy metal is a heat treatment which is normally performed in the fabrication of a semiconductor device. Since this heat treatment is performed several times, however, even if a pollution heavy metal is caught in the substrate 301 in a single heat treatment, the caught pollution heavy metal may be set free again in the next heat treatment. As apparent from the above, the use of the conventional fabrication method cannot remove a pollution heavy metal from the substrate 301 sufficiently.